The sulfur –oxidizing bacteria are chemolithotrophs, they use sulfur oxidation as a means of gaining energy or electrons. The sulfur-oxidizing bacteria can oxidize hydrogen sulfide, polysulfides, and elemental sulfur. These compounds can be found in nature under various aerobic and anaerobic conditions. Sulfur -oxidizing bacteria can be found in geological materials such as volcanic gases and deposits of elemental sulfur. In most cases, oxygen is the preferred electron acceptor. When certain bacteria derive energy from the oxidation of sulfide, the populations of bacteria grow, and in return these bacteria provide food for nearby animals such as bivalve mollusks, which actually allow sulfur-oxidizing bacteria to reside within their gills. Some of these bacteria live in mats on rocky surfaces and on soft sediments, providing a source of food for crabs and bivalves. Sulfide may be oxidized to elemental sulfur by species of Thiothrix and anaerobically by the purple sulfur bacteria. Some eukaryotes have sulfide binding proteins, which allow them to transport sulfide and oxygen to the bacteria through specific organelles such as the trophosome found in hydrothermal vent tubeworms.
Sometimes natural gas deposits contain high concentrations of hydrogen sulfide and thus, create an environment where sulfur-oxidizing bacteria can thrive. Sulfur oxidizers are also found in low pH environments such as acid mine drainage waters. These acid-loving sulfur-oxidizing bacteria called (acidophiles) metabolize sulfur in sulfur-rich coals. When sulfur-rich coals or minerals like pyrite (fool's gold=FeS
2
) are exposed to oxygen, the acidophilic, sulfur-oxidizing bacteria can convert sulfur to sulfuric acid and create an environment with very low pH(~1-2 or lower). Thiobacillus species are often found in both marine and fresh water sediments, other species of sulfur-oxidizing bacteria such as Beggiatoa can grow on sulfide, which is confirmed by the presence of intra-or extra sulfur granules in the cell.
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Importantly, hydrogen sulfide is normally toxic to aerobic plants and animal tissues, with the exception of ruminant animals, and whenever sulfur is generated, a specialized micro flora develops that is capable of oxidizing the sulfide into elemental sulfur.
CO
2
+2H
2
S→ CH
2
O + 2S + H
2
O
The oxidation of sulfur is shown below
2S +3O
2
+ 2HOH→ 2H
2
SO
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(Thiobacillus thiooxidns)
